A technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to the transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a transistor in which an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) and having an electron carrier concentration of lower than 1018/cm3 is used for a semiconductor layer which is used for a channel formation region of the transistor is disclosed (see Patent Document 1).